The present invention relates to a monolithic ceramic gas diffuser for injecting gas into a molten metal bath, and more particularly relates to such a diffuser that includes a portion through which injection gas will preferentially flow.
When making metal and metal alloy products, it is often necessary to create a bath of molten metal that will later be cast into molds of various shapes and sizes. With certain specific alloys, such as aluminum alloys, the molten aluminum is highly sensitive to the presence of hydrogen gas, which tends to form voids in the cast product. Additionally, molten aluminum oxidizes freely when exposed to air, and the resultant aluminum oxide has a density very similar to the metal itself This results in aluminum oxide being suspended in the melt, causing "hard spots" upon solidification, an undesirable result.
In an attempt to prevent both problems, it is conventional to inject a "cleansing gas" such as argon, nitrogen, chlorine, or freon into the molten aluminum in the form of gas bubbles. The hydrogen in the molten aluminum is either absorbed or attaches to the cleansing gas bubbles, which rise to and exit from the surface of the molten aluminum. Additionally, any aluminum oxide suspended in the molten aluminum can be floated to the surface by the gas bubbles. This is a mechanical process, and is basically independent of the type of gas used.
FIG. 1 shows a holding box 1 that contains molten metal 2 therein. Gas injection nozzles or spargers 3 are located at various positions in communication with the molten metal to inject gas, supplied from a gas supply line 4, into the molten metal. FIG. 2 shows an example of an existing sparger that typically would be positioned in the floor of holding box 1. The sparger 5 includes a highly permeable ceramic member 6 encased in a steel can 7 through an interposed refractory or mortar adhesive 8. A gas supply pipe 9 supplies gas to the permeable ceramic member 6 to inject gas into the molten metal.
The problem with the sparger shown in FIG. 2 is that it requires the presence of steel can 7 to encase the permeable ceramic member 6 to insure that gas bubbles are injected only through the end face of permeable ceramic member 6 into the molten metal. Consequently, the sparger shown in FIG. 2 is relatively expensive to manufacture. Moreover, the sparger is susceptible to cracking at the interfaces between permeable ceramic member 6, mortar 8 and steel can 7, due to the differences in thermal expansion coefficient among the various materials. Still further, the permeable ceramic member 6 used in such conventional spargers have large pore size, generally greater than 30 microns in diameter, and thus the size of the gas bubbles injected into the molten metal is relatively large. It would be preferred to inject smaller gas bubbles as they would be more effective in removal of the hydrogen gas contained in the molten metal, thus requiring less gas volume to accomplish degassing of the molten metal.
FIG. 3 shows another example of a gas injection mechanism in the form of a generally cylindrical graphite lance 10. The lance is immersed in the molten metal and gas is introduced through the relatively large opening 11 formed in the end of the lance. The problem with such graphite or other ceramic lances is similar to the problem associated with the sparger shown in FIG. 2, in that it is difficult to inject small gas bubbles into the molten metal using such a device. Moreover, graphite tends to oxidize and corrode, and is also rather fragile; thus it requires frequent replacement.
FIG. 4 shows an example of a rotary degasser developed by Blasch Precision Ceramics, Inc. The rotary degasser 12 includes an elongate shaft 13 having an axial bore 14 extending therethrough, and an impeller 15 integrated with one end of shaft 13. The impeller has a plurality of blades 16 extending radially outwardly from the axis of shaft 13, and gas ports 14a passing radially outwardly through the impeller. The rotary degasser is immersed in molten metal and rotated by a drive member (not shown) while gas is injected into the molten metal through ports 14a and a large opening 17 formed in the end face of impeller 15. Rotation of the impeller facilitates mixing of the injected gas with the molten metal. The problem with this rotary degasser, however, is that the size of the gas bubbles introduced into the molten metal is still quite large, and thus relatively inefficient for degassing the molten metal.
It would be desirable to provide a gas diffuser that is (1) highly resistant to cracking due to thermal cycling and other factors encountered during molten metal manufacturing, (2) capable of injecting uniform, relatively small gas bubbles into a bath of molten metal, and (3) relatively easy and inexpensive to manufacture. The gas diffusers to date, however, have not been able to fulfill all of these requirements.